Multiuser detection aided multiple access differential frequency-hopped spread spectrum

ABSTRACT

Techniques for multiple access differential frequency-hopped spread spectrum (MA-DFHSS) aided by multiuser detection (MUD) are disclosed. An initial DFH decoding is performed to provide data estimates for each user represented in a received co-channel signal. Interference cancellation is then performed using MUD, thereby providing an interference-cancelled signal. Data estimates remaining are then re-decoded. Iteration on the interference cancellation and re-decoding can be carried out to satisfy a particular rule of iteration, although iteration is not always necessary. The final decoded signal can then be provided to its destination.

This application claims the benefit of U.S. Provisional Application No.60/465,026, filed Apr. 24, 2003. In addition, this application is acontinuation-in-part of U.S. application Ser. No. 10/422,340, filed Apr.24, 2003, and entitled “Soft-Decision Trellis-Coded DifferentialFrequency-Hopped Spread Spectrum (DFHSS).” Each of these applications isherein incorporated in its entirety by reference.

FIELD OF THE INVENTION

The invention relates to communications, and more particularly, toco-channel communications in multiple access differentialfrequency-hopped spread spectrum applications.

BACKGROUND OF THE INVENTION

Spread spectrum is a communications technique where the baseband signalbandwidth is intentionally spread over a larger bandwidth by modulatingthe signal with a higher-frequency “spreading” code prior totransmission. As a result, energy used in transmitting the basebandsignal is spread over a wider bandwidth, and appears noiselike. Theratio between the spread baseband and the original signal is calledprocessing gain. Typical Spread spectrum processing gains vary between10 dB and 60 dB. A complementary de-spreading operation is performed atthe receiver. Example spread spectrum techniques include frequencyhopping (FH) and direct sequence spread spectrum (DSSS), each of whichprovide a reliable communications method for users.

In the presence of many users, however, co-channel interference degradesthe performance of existing spread spectrum techniques. Media accesscontrol (MAC) must be implemented to reduce co-channel interference andto coordinate access to the channel by all interested, cooperatingparties. In addition, it is desirable in some applications that thetransmitted waveform has a low probability of detection (LPD) byunintended receivers, and that the transmitted waveform is resilient inthe presence of jamming signals (i.e., AJ).

None of the existing spread-spectrum waveforms provide a signal that hassuitable LPD and AJ performance, while simultaneously allowing forconferencing without a MAC, and easy implementation. In addition, eachknown technique is associated with one or more disadvantages, such aswasteful use of bandwidth in DSSS, performance degradation in thepresence of burst errors, and performance degradation in the presence ofinterference, both hostile and non-hostile.

The differential frequency-hopped (DFH) spread spectrum waveform anddecoder combines trellis encoding concepts and frequency hopping (FH)techniques to provide improved LPD and AJ performance when compared toexisting spread spectrum techniques. In addition, DFH spread spectrumtechniques allow multiple users without a MAC and are relatively easy toimplement.

What is needed, therefore, are techniques to implement a conferenced,multiple access operation of differential frequency-hopping, bycombining both DFH and an iterative form of multiuser detection (MUD).This approach is extendible from frequency hopping to any M-ary codingscheme not fully utilizing its capabilities.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention provides a method for receiving(in a multiuser communication environment) a co-channel signal includinga target-user differential frequency hopped (DFH) signal and one or moreinterfering other-user DFH signals. The method includes decoding the DFHsignal, thereby providing soft-decision estimates of data bits includedin the co-channel signal for each user. The method continues withremoving estimated contributions of the interfering other-user DFHsignals using multiuser detection (MUD), thereby providing aninterference-cancelled signal. The method proceeds with re-decodingambiguous data estimates remaining in the interference-cancelled signal.

The method may further include repeating the removing and re-decodingone or more times. Alternatively, or in addition to, the method mayfurther include providing the re-decoded data estimates as final decodeddata. In one particular embodiment, decoding the DFH signal includesdetecting at least one frequency per a predetermined time interval ofthe co-channel signal, constructing a trellis model for each user usingdetected frequencies as nodes, and determining soft-decision estimatesof data bits included in the co-channel signal for each user, using thecorresponding trellis model.

Another embodiment of the present invention provides a method forreceiving a differential frequency hopped (DFH) signal in a multiusercommunication system. The method includes receiving a co-channel signalincluding a target-user DFH signal and one or more interferingother-user DFH signals, detecting at least one frequency per apredetermined time interval of the co-channel signal, constructing atrellis model for each user using detected frequencies as nodes, anddetermining soft-decision estimates of data bits included in theco-channel signal for each user, using the corresponding trellis model.This particular embodiment further includes removing estimatedcontributions of the interfering other-user DFH signals using multiuserdetection (MUD), thereby providing an interference-cancelled signal. Themethod continues with re-decoding ambiguous data estimates remaining inthe interference-cancelled signal.

In response to determining iteration is likely to improve the quality ofthe re-decoded data estimates, the method may further include repeatingthe removing and re-decoding. In response to determining iteration isnot likely to improve the quality of the re-decoded data estimates,however, the method may include providing the re-decoded data estimatesas final decoded data. Alternatively, the method may simply includerepeating the removing and re-decoding one or more times, and/orproviding the re-decoded data estimates as final decoded data.

In one particular embodiment, determining the soft-decision estimatesincludes generating estimates of the data bits based on a cumulativesoft-valued metric, and/or providing a confidence value for eachestimate. In another particular embodiment, the method further includesinferring missing nodes of the trellis model from existing nodes basedon detected frequencies, and correcting for burst errors.

Another embodiment of the present invention provides a system (MUD-aidedDFH receiver) for receiving in a multiuser communication environment aco-channel signal including a target-user differential frequency hopped(DFH) signal and one or more interfering other-user DFH signals. Thesystem includes an initial DFH decoding module and an interferencecancellation and re-decoding module. The initial DFH decoding module isadapted to detect at least one frequency per a predetermined timeinterval of the co-channel signal, thereby enabling construction of atrellis model for each user using detected frequencies as nodes, and isalso adapted to determine soft-decision estimates of data bits includedin the co-channel signal for each user, using the corresponding trellismodel. The interference cancellation and re-decoding module isoperatively coupled to the initial DFH decoding module, and is adaptedto remove estimated contributions of the interfering other-user DFHsignals using MUD, thereby providing an interference-cancelled signal,and is also adapted to re-decode ambiguous data estimates remaining inthe interference-cancelled signal.

In one such embodiment, the initial DFH decoding module includes afrequency detector for detecting the at least one frequency per apredetermined time interval of the co-channel signal, and one or moresoft decision trellis decoders for determining the soft-decisionestimates of data bits. Each soft decision trellis decoder can befurther adapted, for example, to generate estimates of the data bitsbased on a cumulative soft-valued metric, and/or to provide a confidencevalue for each estimate. Each soft decision trellis decoder can befurther adapted to infer missing nodes of trellis model from existingnodes based on the detected frequencies, and to correct for bursterrors.

In another such embodiment, the interference cancellation andre-decoding module includes one or more multiuser detectors for removingthe estimated contributions of the interfering other-user DFH signalsfor each user, and a corresponding soft decision trellis decoderoperatively coupled to each multiuser detector, for re-decoding theambiguous data estimates remaining in the interference-cancelled signal.

In another such embodiment, the system further includes an iterationcontroller that is operatively coupled to the interference cancellationand re-decoding module, and is adapted to provide the re-decoded dataestimates for further processing by the interference cancellation andre-decoding module when appropriate, based on an iteration rule. Inresponse to determining that iteration is not likely to improve thequality of the re-decoded data estimates, the iteration controller canprovide the re-decoded data estimates as final decoded data.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a functional block diagram of a DFH transmitterconfigured to operate in accordance with an embodiment of the presentinvention.

FIG. 2 is a graph illustrating an example trellis model for thetransmitter illustrated in of FIG. 1.

FIG. 3 is a block diagram illustrating a DFH receiver following onetrellis, in a single user system.

FIG. 4 is a block diagram illustrating a soft decision trellis decodingprocess in accordance with an embodiment of the present invention.

FIG. 5 is a functional block diagram of a MUD-aided DFH receiverconfigured in accordance with an embodiment of the present invention.

FIG. 6 a is a detailed block diagram of a MUD-aided DFH receiverconfigured with no iteration in accordance with an embodiment of thepresent invention.

FIG. 6 b is a detailed block diagram of a MUD-aided DFH receiverconfigured with iteration in accordance with an embodiment of thepresent invention.

FIG. 7 is a flow chart illustrating a method for receiving a DFH signalin a multiuser communication system in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention implement a conferenced, multipleaccess operation of differential frequency-hopping, by combining bothDFH and an iterative form of multiuser detection (MUD). The describedapproach is extendible from frequency hopping to any M-ary coding schemenot fully utilizing its capabilities. For instance, it can be extendedto hopping in time, code, phase, dwell time, etc., as well as hoppingfrequency.

Interfering DFH signals are decoded, thus removing the frequencydetections that correspond to confidently decoded interfering symbolsfrom the time-frequency detection matrix, and re-decoding the result.This process may continue iteratively until a iteration controldetermines that iterating is to stop (e.g., based on a predefined ruleof iteration), then the decoded symbols for the desired user are output.The conferencing multiple access capabilities of the DFH spread spectrumwaveform are significantly improved by using iterative multiuserdecoding techniques.

DFH Transmitter

For DFH waveforms, the frequency of the transmitted tone depends on boththe current data symbol and the previous transmitted tone. Thus, given adata symbol X_(n) and frequency of the previous hop F_(n-1), thefrequency of the next hop is determined as: F_(n)=G(F_(n-1), X_(n)),where the function G can be viewed as a directed graph which has nodescorresponding to frequencies, and vertices labeled with input datapatterns. Because the series of transmitted tones form a trellis, thereceiver is able to make soft decisions on the data bit estimates, whichprovides for improved performance. Also, the waveform becomes difficultfor unintended receivers to decode.

It has been shown that a communication system using a DFH-based waveformand trellis-based receiver outperforms standard frequency hopping (FH)and direct sequence spread spectrum (DSSS) under a variety ofconditions, independent of frequency band. For instance, the receivercan reconstruct hops that are missing due to a fading channel or fromcollisions with other users. The trellis also allows for conferencing ofmultiple users for receivers that follow multiple trellises. Attempts tojam the waveform by fast-follow-on jammers can actually increase thestrength of the received signal and improve performance of a DFH-basedsystem.

FIG. 1 illustrates a functional block diagram of a DFH transmitterconfigured in accordance with an embodiment of the present invention.The transmitter includes a b-bit buffer 110, a tone selector G(.) module130, a number of delay elements 150, and a tone generator module 170.The transmitted tones form a trellis defined by the function G, aspreviously explained.

The data bit stream from a data source is passed on input line 100 tothe b-bit buffer 110, which collects b bits of the data stream. Thevalue of b is the number of bits that are encoded within each frequencyhop. The b-bit symbol is passed on line 120 to the tone selector G(.)module 130, which uses both the current b bits of data and at least onepreviously transmitted tone from line 160 to determine the currentfrequency selection. The current frequency selection is provided on line140 to the tone generator module 170, which transmits the selected toneon line 180. The tone selection on line 140 is also passed to a delaymodule 150, for use in subsequent tone selections.

Each of these components can be conventionally implemented in hardware,software, firmware, or some combination thereof. For example, eachmodule can be implemented as a set of software instructions executing ona digital signal processor or other suitable processing environment.

Trellis models, often used in depicting and analyzing convolutionalcodes, can readily be applied to a differential frequency-hopped signal,as shown in FIG. 2. While trellis models are the rule-based functionused here to describe the frequency (tone) selection process in DFH, anyother type of rule representation is also possible. As can be seen inFIG. 2, the vertical axis of the trellis corresponds to frequency, whilethe horizontal axis corresponds to time intervals. The set of states atany given time corresponds to the set of all possible frequencies thatmay be transmitted by the DFH system.

For a hopset of size M, there are M possible states at each stage in thetrellis. The branches leaving each state terminate at the frequenciesthat are possible at the next hop given the current frequency state. Alabel on each branch indicates the encoded bits that corresponds to thetransition from the current transmitted frequency to the nexttransmitted frequency.

For the trellis in FIG. 2, b=1 bit/hop, hopset size M=4, and the datasequence shown by the dotted line is 0110. Note that the first detectionat frequency 3, F3, corresponds to a 0 data bit and the second detectionat F3 corresponds to a 1 data bit, which illustrates the DFH featurethat the sequence of detections carry the information, and not thedetections themselves, per se.

While FIG. 2 shows two axes, the number of axes for any given system isdependent on the number of identifying features (e.g., phase, amplitude,dwell time, duty cycle) that the system designer chooses to track.Although the description here specifically details transmitting data anddetermining a frequency value based on the current data and previousfrequency values, the concept can be extended to apply to otherproperties or combinations of properties of the signal, including butnot limited to, phase, coding, dwell, and duty cycle.

DFH Receiver—One Trellis

At the receiving node, the transmitted signal is received and passed toa frequency detector, where at least one frequency per a predeterminedtime interval is determined. The detected frequency is passed to a softdecision decoder that constructs a trellis model using the detectedfrequencies as nodes, estimates a data value according to the knowntrellis constraints of a transmitted signal, and outputs a dataestimate.

FIG. 3 shows a DFH receiver following one trellis (i.e., decoding onetransmitted signal). The receiver includes a frequency detector module310 and a soft decision trellis decoder module 330. As can be seen, thereceived signal is passed on line 300 to the frequency detector module310 (such as a spectrogram) that determines which frequency orfrequencies are present during each time interval. The set of detectedfrequencies is provided at line 320 and passed to the trellis decodermodule 330, which determines a soft-decision estimate of the transmitteddata bits. This estimate is then provided at line 340.

The technique used in the trellis decoder 330 may be any trellisdecoding techniques, such as those employed by Viterbi or maximum aposteriori (MAP) decoders. The decoder 330 generates estimates of thedata sequence based on a cumulative soft-valued metric. For instance, asoft-Viterbi decoder may be used which places the detected frequenciesat each symbol interval on a trellis construction, and then the decoder330 attempts to “connect” the detected frequencies according to knowntrellis constraints of the transmitted signal.

Burst errors are correctable because missing nodes on the trellis may beinferred from the existing nodes on the trellis (on both sides of themissing node or nodes), due to the trellis-encoded nature of thetransmitted signal. Soft-decision decoding provides an estimate of thedemodulated symbol as well as a confidence value for that estimate, andafter some delay, the decoder 330 chooses the actual estimate. Thisprocess generally improves performance when compared to other modulationtypes and to hard-decision techniques.

DFH Receiver—Multiuser System

For a system in which the receiver is interested in demodulating signalsfrom several users, the soft decision decoding is replicated for eachdesired signal. FIG. 4 is a block diagram illustrating a soft decisiontrellis decoding process replicated for each desired signal inaccordance with an embodiment of the present invention. Here, afrequency detector module 410 is operatively coupled by line 420 to anumber (N) of decoder modules 430.

Each decoder 430 represents the soft-decision trellis decoding processthat is followed for each desired user (users 1 through N). Each decoder430 is functionally identical, but decodes the received signal by usingthe trellis of the transmitting user of interest. In particular, adifferent trellis rule in accordance with the function G_(x)(.), aspreviously discussed. This trellis decoding process is further describedin detail in related U.S. application Ser. No. 10/422,340, entitled“Soft-Decision Trellis-Coded Differential Frequency-Hopped SpreadSpectrum (DFHSS).”

FIG. 5 is a functional block diagram of a MUD-aided DFH receiverconfigured in accordance with an embodiment of the present invention.The receiver includes an initial DFH decoder module 510, an interferencecancellation and MA-DFH re-decode module 530, and an iterationcontroller module 550. It will be appreciated in light of thisdisclosure that a general MUD-aided DFH multiaccess (MA) transmittermodel operates as a DFH transmitter. It will further be appreciated thata MUD-aided DFH approach in accordance with the principles of thepresent invention is best demonstrated and understood at the receiver,as is shown in FIG. 5.

As can be seen in FIG. 5, the received signal is provided on line 400 tothe initial DFH receiver module 510, which detects all frequencydetections for each time interval and trellis-decodes the receivedfrequency detections for each trellis that is known or might bepotentially used to generate DFH sequences. Thus, the initial DFHreceiver module 510 decodes the signal of interest as well asinterfering DFH signals of all known-trellis users. Note that, in theembodiment illustrated, the process carried out by initial DFH receivermodule 510 is represented in FIG. 4, and is described in detail in U.S.application Ser. No. 10/422,340.

For optimal interference cancellation at the later stages, a separateDFH detector can operate on the received signal for each active user,but improved performance will be seen even if only a subset ofinterfering transmissions are removed. After the initial DFH detectionis performed by module 510, the successfully decoded symbols frominterfering users are identified on line 435 and their contributions tothe received matrix are removed in the interference cancellation sectionof module 530.

The MA-DFH section of module 530 then uses the modified received matrixto re-decode the ambiguous received values for the remaining detectionsin the time-frequency matrix. The updated decoded values are provided online 540. If the iteration controller module 550 determines thatiterating is not likely to improve the quality of the signal, theiterating process stops, and the symbols are output on line 565 as thefinal values. If the iteration controller module 550 determines thatfurther iterations might be useful, the updated decoded values arepassed on line 560 back to module 530, which again removes the frequencydetections corresponding to successful decoded values for all of theinterfering users, and re-decodes.

The iteration controller module 550 might, for example, count the numberof iterations and stop after a pre-determined value, or it might compareprevious decoded values to current decoded values and stop iterating ifno changes were made, or it might examine confidence values associatedwith the soft decoding done in module 530. Other predefined rules ofiteration can be employed by module 550 as well. Further note that theiteration controller can also be configured to simply pass the decodedvalues through to the output at line 565 (if no iteration is desired).In any event, the decoded values can be effectively switched to adesired line (e.g., feedback output line and final value output line).

FIG. 6 a is a detailed block diagram of a MUD-aided DFH receiverconfigured with no iteration in accordance with an embodiment of thepresent invention. As previously discussed, the receiver includes aninitial DFH decoder module 510 and an interference cancellation andMA-DFH re-decode module 530. There is no iteration controller moduleemployed in this embodiment. Alternatively, although there may be aniteration controller, only a single iteration is carried out (e.g.,where the iteration controller is configured as a feed-through to output565, thereby reducing processing time due to iteration).

In this particular example, the signal received at line 400 includesboth a target signal (e.g., user 1) and two interfering signals (e.g.,users 2 and 3). The received signal is provided to the frequencydetector module 410 that determines which frequency or frequencies arepresent during each time interval. In one embodiment, the frequencydetector module 410 is configured as a spectrogram. Other knownfrequency detection techniques can be employed here as well.

The set of detected frequencies is provided at line 420 to a bank oftrellis decoder modules 430, where there is one trellis decoder for eachreceived user. Each trellis decoder module 430 determines asoft-decision estimate of the corresponding transmitted data bit. Eachestimate is then provided at the corresponding line 435. It will beappreciated that the soft decision trellis decoding for each user can beperformed in parallel by distinct decoder modules as shown, or one at atime with a single decoder module. The previous discussions on trellisdecoder embodiments and functionality in reference to FIGS. 4 and 5equally apply here.

After the initial DFH detection is performed by module 510, thesuccessfully decoded symbols from interfering users provided on line 435are received at the interference cancellation and re-decoding module530. Here, the decoded symbols from interfering users are received atrespective interference cancellation (IC) modules 640. Each IC module640 is adapted to remove the decoded symbols contributed to the receivedmatrix by interfering users.

Conventional other-user interference cancellation techniques can beemployed here (e.g., turboMUD or other known MUD algorithms). In oneparticular embodiment, the same-system interference cancellationtechniques described in U.S. application Ser. No. (not known yet), filedJun. xx, 2003, and entitled “Cross-System Interference Cancellation forMulticarrier CDMA and OFDM” are employed by each module 640. Thisapplication is herein incorporated by reference in its entirety.

Each interference cancellation module 640 provides aninterference-cancelled matrix at a corresponding line 645. Acorresponding soft decision trellis decoder module 650 then uses themodified matrix to re-decode the ambiguous received values for theremaining detections in the time-frequency matrix. The updated decodedvalues are provided on a respective line 540. As no iteration isemployed here, these decoded values are final, and can be provided totheir intended destinations (e.g., such as a local host or network).

FIG. 6 b is a detailed block diagram of a MUD-aided DFH receiverconfigured with iteration in accordance with an embodiment of thepresent invention. The operation here is similar to that discussed inreference to FIG. 6 a, except that the updated decoded values output onlines 540 are provided to the iteration controller module 550, whichoperates pursuant to an iteration rule 550 a. The iteration controllermodule 550 decides according to the iteration rule 550 a whether to passthe data estimates back for more MUD and re-decoding on lines 560, or tooutput the data estimates as final estimates on lines 565. The previousdiscussions relevant to the iteration controller and iteration ruleequally apply here.

Note that the feedback path allows the input to the modules 640 to beeffectively switched from lines 435 to lines 560 so that iterativeprocessing and refinement of the data estimates can take place. Duringsuch iteration, note that the a “next” set of estimates provided onlines 435 by the initial DFH decoder module 510 can be buffered orotherwise preserved until the iterative processing of the “current”estimates are finalized by the MA-DFH re-decode module 530.

Each of the components illustrated in FIGS. 6 a and 6 b can beconventionally implemented in hardware, software, firmware, or somecombination thereof. For example, each module can be implemented as aset of software instructions executing on a digital signal processor orother suitable processing environment. Alternatively, each module can beimplemented in purpose built silicon, such as one or more ASICsconfigured to provide the described functionality. Alternatively, thedescribed finctionality can be coded on a processor readable medium(e.g., such as a server, disk, or other computer program product) as oneor more routines.

Embodiments of the present invention provide a number of advantages,including improved bit error rate (BER) performance for the same numberof users, when compared to DFH. Also, an increase in the number ofallowable users is enabled without affecting the BER performance. Thus,multiuser detection techniques are combined to differential frequencyhopping systems, thereby increasing the number of simultaneous usersthat may operate in the same area without degradation of the decodeddata.

FIG. 7 is a flow chart illustrating a method for receiving a DFH signalin a multiuser communication system in accordance with an embodiment ofthe present invention. This method can be carried out, for example, bythe receiver discussed in reference to FIGS. 6 a or 6 b.

As can be seen, the method includes an initial DFH detection portion, aswell as an interference cancellation and DFH re-decoding portion. Theinitial DFH detection portion of the method begins with receiving 705 aco-channel signal including a target-user DFH signal and one or moreinterfering other-user DFH signals. The method proceeds with detecting710 at least one frequency per a predetermined time interval of theco-channel signal, and constructing 715 a trellis model for each userusing the detected frequencies as nodes. The initial DFH detectionportion of the method continues with determining 720 soft-decisionestimates of data bits included in the co-channel signal for each user,using the corresponding trellis model.

The interference cancellation and DFH re-decoding portion then proceedswith removing 725 estimated contributions of the interfering other-userDFH signals using MUD (e.g., conventional multiuser detection techniquesmay be employed here, such as turboMUD), and re-decoding 730 ambiguousdata estimates remaining in the interference-cancelled signal. Adetermination 730 can then be made at to whether iteration is likely toimprove the quality of the estimated signal. As previously explained,when iteration is employed, an rule of iteration can be used in thedetermination. If iteration is required, then the data estimates aresubjected to the interference cancellation and re-decoding of steps 725and 730. Otherwise, the iterating process stops, and the final decodeddata is provided to its intended destination.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

1. A method for receiving a differential frequency hopped (DFH) signalin a multiuser communication system, the method comprising: receiving aco-channel signal including a target-user DFH signal and one or moreinterfering other-user DFH signals; detecting at least one frequency pera predetermined time interval of the co-channel signal; constructing atrellis model for each user using detected frequencies as nodes;determining soft-decision estimates of data bits included in theco-channel signal for each user, using the corresponding trellis model;removing estimated contributions of the interfering other-user DFHsignals using multiuser detection (MUD), thereby providing aninterference-cancelled signal; and re-decoding ambiguous data estimatesremaining in the interference-cancelled signal.
 2. The method of claim 1further comprising: in response to determining iteration is likely toimprove the quality of the re-decoded data estimates, repeating theremoving and re-decoding.
 3. The method of claim 1 further comprising:in response to determining iteration is not likely to improve thequality of the re-decoded data estimates, providing the re-decoded dataestimates as final decoded data.
 4. The method of claim 1 furthercomprising: repeating the removing and re-decoding one or more times. 5.The method of claim 1 further comprising: providing the re-decoded dataestimates as final decoded data.
 6. The method of claim 1 whereindetermining the soft-decision estimates includes generating estimates ofthe data bits based on a cumulative soft-valued metric.
 7. The method ofclaim 1 wherein determining the soft-decision estimates includesproviding a confidence value for each estimate.
 8. The method of claim 1wherein further comprising: inferring missing nodes of the trellis modelfrom existing nodes based on detected frequencies; and correcting forburst errors.
 9. A system for receiving in a multiuser communicationenvironment a co-channel signal including a target-user differentialfrequency hopped (DFH) signal and one or more interfering other-user DFHsignals, the system comprising: an initial DFH decoding module adaptedto detect at least one frequency per a predetermined time interval ofthe co-channel signal, thereby enabling construction of a trellis modelfor each user using detected frequencies as nodes, and to determinesoft-decision estimates of data bits included in the co-channel signalfor each user, using the corresponding trellis model; and aninterference cancellation and re-decoding module operatively coupled tothe initial DFH decoding module, and adapted to remove estimatedcontributions of the interfering other-user DFH signals using multiuserdetection (MUD) thereby providing an interference-cancelled signal, andto re-decode ambiguous data estimates remaining in theinterference-cancelled signal.
 10. The system of claim 9 wherein theinitial DFH decoding module includes: a frequency detector for detectingthe at least one frequency per a predetermined time interval of theco-channel signal; and one or more soft decision trellis decoders fordetermining the soft-decision estimates of data bits.
 11. The system ofclaim 10 wherein each soft decision trellis decoder is further adaptedto generate estimates of the data bits based on a cumulative soft-valuedmetric.
 12. The system of claim 10 wherein each soft decision trellisdecoder is further adapted to provide a confidence value for eachestimate.
 13. The system of claim 10 wherein each soft decision trellisdecoder is further adapted to infer missing nodes of trellis model fromexisting nodes based on the detected frequencies, and to correct forburst errors.
 14. The system of claim 9 wherein the interferencecancellation and re-decoding module includes: one or more multiuserdetectors for removing the estimated contributions of the interferingother-user DFH signals for each user; and a corresponding soft decisiontrellis decoder operatively coupled to each multiuser detector, forre-decoding the ambiguous data estimates remaining in theinterference-cancelled signal.
 15. The system of claim 9 furthercomprising: an iteration controller operatively coupled to theinterference cancellation and re-decoding module, and adapted to providethe re-decoded data estimates for further processing by the interferencecancellation and re-decoding module when appropriate, based on aniteration rule.
 16. The system of claim 15 wherein in response todetermining that iteration is not likely to improve the quality of there-decoded data estimates, the iteration controller provides there-decoded data estimates as final decoded data.
 17. A method forreceiving in a multiuser communication environment a co-channel signalincluding a target-user differential frequency hopped (DFH) signal andone or more interfering other-user DFH signals, the method comprising:decoding the DFH signal, thereby providing soft-decision estimates ofdata bits included in the co-channel signal for each user; removingestimated contributions of the interfering other-user DFH signals usingmultiuser detection (MUD), thereby providing an interference-cancelledsignal; and re-decoding ambiguous data estimates remaining in theinterference-cancelled signal.
 18. The method of claim 17 furthercomprising: repeating the removing and re-decoding one or more times.19. The method of claim 17 further comprising: providing the re-decodeddata estimates as final decoded data.
 20. The method of claim 17 whereindecoding the DFH signal includes: detecting at least one frequency per apredetermined time interval of the co-channel signal; constructing atrellis model for each user using detected frequencies as nodes; anddetermining soft-decision estimates of data bits included in theco-channel signal for each user, using the corresponding trellis model.